Factual error: After returning to the Dursley's after the dementor attack It is dark, this shows again when Vernon puts Dudley in the car. However, the howler from Mathila Hopkirk says that at 6:23pm Harry performed illegal magic. As the side story shows it is a hot summer, it wouldn't get dark until way past 8pm. (00:04:53 - 00:07:20)
Continuity mistake: After the battle in the Hall of Prophecy, when Harry and the others land in the Death Chamber, Ginny is a good bit of distance away from Harry's feet in the side shot as they're all lying on the floor, but two shots later as they're starting to get up Ginny's head is now right beside Harry's knee. (01:52:25)
Continuity mistake: When Harry is narrating a letter to Padfoot, he sees a Thestral flying in the far distance, and it disappears behind a mountain. He hikes (presumably) all the way to where he thinks the Thestral went, and he finds Luna Lovegood in a clearing with a few Thestrals. For one thing, the Thestral that disappeared behind the mountain was MILES away, and it would have taken Harry a couple of hours, at least, to hike that distance through mountainous terrain. But, when he arrives at the clearing with the Thestrals, the daylight hasn't changed at all (should have been noticeably different lighting). Once he arrives there, he sees that even the largest Thestral is no bigger than a horse. But, when he first saw the Thestral disappear behind the mountain, at that distance the Thestral was about the size of a commercial jet airliner.
Continuity mistake: At the playground, when Harry sits on the swing his long shadow is cast directly in front of him as he watches a boy and his mother by the round-a-bout, which is also directly in front of him. Problem is their shadows, which are considerably shorter than Harry's, are cast to Harry's left. (00:01:05)
Continuity mistake: When the Death Eaters whirl around Harry (grabbing the other students) in the Death Chamber, near the Veil, in the wideshots he is lying on the ground with his head upon his right arm, but in the previous and following close-ups his arm is lower and his head is lying directly on the ground. (01:53:20)
Continuity mistake: When Harry and Dudley reach the tunnel (out of the rain) Harry stands level with the "r" and "k" in the very large graffitied word "Stark", and Dudley is well past the large white graffiti to his right (viewer's left). The next shot, from the boys' POV, the empty tunnel is shown and on the left is the gratified word "Stark" and part of the white graffiti on the left, even though the two boys were actually just standing quite a bit farther into that tunnel. (00:03:00)